Because lights of the LEDs are emitted from an active layer therein and head toward all directions, the emission efficiency of LEDs can be represented in the following formula:Eeff=Ei(internal)*Ee(external)
Wherein Ei is the emission efficiency of the internal active layer; Ee is the emission efficiency of the externally emitted lights, while Eeff is the overall emission efficiency.
The attempts of improving emission efficiency of LEDs in prior arts have been mostly concentrated on improving the emission efficiency of the externally emitted lights, as can be observed in U.S. Patents US20050082562A1, US20050277218A1, US20040104672, U.S. Pat. No. 6,900,473, and US67777871. All of which are focused on improving emission efficiency of LEDs by modifying the surface of LEDs, so that irregular surfaces may be formed externally on the LEDs (via the method of etching, for example), thus preventing lights of the LEDs from total internal reflection and enhancing the emission efficiency of the externally emitted lights Ee. However, the level of Ee has reached approximately 80% under the currently available technologies, and is already straining close to physical limitations.